A conventional high resolution imaging Synthetic Aperture Radar (SAR) flown on a spacecraft such as a satellite operates by transmitting pulses of electromagnetic radiation at regular intervals towards the surface of a planet such as Earth being imaged. Then basically it measures the time required for different parts of the echo to return from the surface and from this creates an image of the swath of surface to which the pulses have been transmitted. The swaths which are resolved are long narrow strips on the ground.
Such a conventional high resolution imaging Synthetic Aperture Radar (SAR) flown on a spacecraft such as a satellite has a major disadvantage when used for planet surveillance purposes. This is because the imaged swath width is limited by theoretical considerations to approximately 150 km for a 15 m antenna as antennas longer than this are very difficult to build in space. For a single spacecraft, such as a satellite, this means that it would take more than two days to cover the surface of the Earth. By scanning the SAR beam of pulses in the across-track direction, it is possible to get some increase of the imaging swath width, but this is directly at the expense of resolution in the along-track direction. It is however not possible to scan out to high incidence angles towards the planet horizon without the swath becoming ambiguous, which is essentially an aliassing problem caused by undersampling, Thus scenes at different ranges become superimposed or folded on top of each other with accompanying problems of resolution.